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.TH GETHOSTBYNAME 3NSL "March 30, 2022"
.SH NAME
gethostbyname, gethostbyname_r, gethostbyaddr, gethostbyaddr_r, gethostent,
gethostent_r, sethostent, endhostent \- get network host entry
.SH SYNOPSIS
.nf
\fBcc\fR [ \fIflag\fR... ] \fIfile\fR... \fB-lnsl\fR [ \fIlibrary\fR... ]
#include <netdb.h>

\fBstruct hostent *\fR\fBgethostbyname\fR(\fBconst char *\fR\fIname\fR);
.fi

.LP
.nf
\fBstruct hostent *\fR\fBgethostbyname_r\fR(\fBconst char *\fR\fIname\fR,
     \fBstruct hostent *\fR\fIresult\fR, \fBchar *\fR\fIbuffer\fR, \fBint\fR \fIbuflen\fR,
     \fBint *\fR\fIh_errnop\fR);
.fi

.LP
.nf
\fBstruct hostent *\fR\fBgethostbyaddr\fR(\fBconst char *\fR\fIaddr\fR, \fBint\fR \fIlen\fR,
     \fBint\fR \fItype\fR);
.fi

.LP
.nf
\fBstruct hostent *\fR\fBgethostbyaddr_r\fR(\fBconst char *\fR\fIaddr\fR, \fBint\fR \fIlength\fR,
     \fBint\fR \fItype\fR, \fBstruct hostent *\fR\fIresult\fR, \fBchar *\fR\fIbuffer\fR,
     \fBint\fR \fIbuflen\fR, \fBint *\fR\fIh_errnop\fR);
.fi

.LP
.nf
\fBstruct hostent *\fR\fBgethostent\fR(\fBvoid\fR);
.fi

.LP
.nf
\fBstruct hostent *\fR\fBgethostent_r\fR(\fBstruct hostent *\fR\fIresult\fR,
     \fBchar *\fR\fIbuffer\fR, \fBint\fR \fIbuflen\fR, \fBint *\fR\fIh_errnop\fR);
.fi

.LP
.nf
\fBint\fR \fBsethostent\fR(\fBint\fR \fIstayopen\fR);
.fi

.LP
.nf
\fBint\fR \fBendhostent\fR(\fBvoid\fR);
.fi

.SH DESCRIPTION
These functions are used to obtain entries describing hosts. An entry can come
from any of the sources for \fBhosts\fR specified in the
\fB/etc/nsswitch.conf\fR file. See \fBnsswitch.conf\fR(5). These functions have
been superseded by \fBgetipnodebyname\fR(3SOCKET),
\fBgetipnodebyaddr\fR(3SOCKET), and \fBgetaddrinfo\fR(3SOCKET), which provide
greater portability to applications when multithreading is performed or
technologies such as IPv6 are used. For example, the functions described in the
following cannot be used with applications targeted to work with IPv6.
.sp
.LP
The \fBgethostbyname()\fR function searches for information for a host with the
hostname specified by the character-string parameter \fIname\fR.
.sp
.LP
The \fBgethostbyaddr()\fR function searches for information for a host with a
given host address. The parameter \fBtype\fR specifies the family of the
address. This should be one of the address families defined in
\fB<sys/socket.h>\fR\&. See the \fBNOTES\fR section for more information. Also
see the \fBEXAMPLES\fR section for information on how to convert an Internet
\fBIP\fR address notation that is separated by periods (.) into an \fIaddr\fR
parameter. The parameter \fIlen\fR specifies the length of the buffer indicated
by \fIaddr\fR.
.sp
.LP
All addresses are returned in network order. In order to interpret the
addresses, \fBbyteorder\fR(3C) must be used for byte order conversion.
.sp
.LP
The \fBsethostent()\fR, \fBgethostent()\fR, and \fBendhostent()\fR functions
are used to enumerate host entries from the database.
.sp
.LP
The \fBsethostent()\fR function sets or resets the enumeration to the beginning
of the set of host entries. This function should be called before the first
call to \fBgethostent()\fR. Calls to \fBgethostbyname()\fR and
\fBgethostbyaddr()\fR leave the enumeration position in an indeterminate state.
If the \fIstayopen\fR flag is non-zero, the system can keep allocated resources
such as open file descriptors until a subsequent call to \fBendhostent()\fR.
.sp
.LP
Successive calls to the \fBgethostent()\fR function return either successive
entries or \fINULL,\fR indicating the end of the enumeration.
.sp
.LP
The \fBendhostent()\fR function can be called to indicate that the caller
expects to do no further host entry retrieval operations; the system can then
deallocate resources it was using. It is still allowed, but possibly less
efficient, for the process to call more host retrieval functions after calling
\fBendhostent()\fR.
.SS "Reentrant Interfaces"
The \fBgethostbyname()\fR, \fBgethostbyaddr()\fR, and \fBgethostent()\fR
functions use static storage that is reused in each call, making these
functions unsafe for use in multithreaded applications.
.sp
.LP
The \fBgethostbyname_r()\fR, \fBgethostbyaddr_r()\fR, and \fBgethostent_r()\fR
functions provide reentrant interfaces for these operations.
.sp
.LP
Each reentrant interface performs the same operation as its non-reentrant
counterpart, named by removing the \fB_r\fR suffix. The reentrant interfaces,
however, use buffers supplied by the caller to store returned results and the
interfaces are safe for use in both single-threaded and multithreaded
applications.
.sp
.LP
Each reentrant interface takes the same parameters as its non-reentrant
counterpart, as well as the following additional parameters. The parameter
\fIresult\fR must be a pointer to a \fBstruct hostent\fR structure allocated by
the caller. On successful completion, the function returns the host entry in
this structure. The parameter \fIbuffer\fR must be a pointer to a buffer
supplied by the caller. This buffer is used as storage space for the host data.
All of the pointers within the returned \fBstruct hostent\fR \fIresult\fR point
to data stored within this buffer. See the \fBRETURN VALUES\fR section for more
information. The buffer must be large enough to hold all of the data associated
with the host entry. The parameter \fIbuflen\fR should give the size in bytes
of the buffer indicated by \fIbuffer\fR. The parameter \fIh_errnop\fR should be
a pointer to an integer. An integer error status value is stored there on
certain error conditions. See the \fBERRORS\fR section for more information.
.sp
.LP
For enumeration in multithreaded applications, the position within the
enumeration is a process-wide property shared by all threads. The
\fBsethostent()\fR function can be used in a multithreaded application but
resets the enumeration position for all threads. If multiple threads interleave
calls to \fBgethostent_r()\fR, the threads will enumerate disjoint subsets of
the host database.
.sp
.LP
Like their non-reentrant counterparts, \fBgethostbyname_r()\fR and
\fBgethostbyaddr_r()\fR leave the enumeration position in an indeterminate
state.
.SH RETURN VALUES
Host entries are represented by the \fBstruct hostent\fR structure defined in
\fB<netdb.h>\fR:
.sp
.in +2
.nf
struct hostent {
    char    *h_name;         /* canonical name of host */
    char    **h_aliases;     /* alias list */
    int     h_addrtype;      /* host address type */
    int     h_length;        /* length of address */
    char    **h_addr_list;   /* list of addresses */
};
.fi
.in -2

.sp
.LP
See the \fBEXAMPLES\fR section for information about how to retrieve a ``.''
separated Internet \fBIP\fR address string from the \fIh_addr_list\fR field of
\fBstruct hostent\fR.
.sp
.LP
The \fBgethostbyname()\fR, \fBgethostbyname_r()\fR, \fBgethostbyaddr()\fR, and
\fBgethostbyaddr_r()\fR functions each return a pointer to a \fBstruct
hostent\fR if they successfully locate the requested entry; otherwise they
return \fINULL\fR.
.sp
.LP
The \fBgethostent()\fR and \fBgethostent_r()\fR functions each return a pointer
to a \fBstruct hostent\fR if they successfully enumerate an entry; otherwise
they return \fINULL\fR, indicating the end of the enumeration.
.sp
.LP
The \fBgethostbyname()\fR, \fBgethostbyaddr()\fR, and \fBgethostent()\fR
functions use static storage, so returned data must be copied before a
subsequent call to any of these functions if the data is to be saved.
.sp
.LP
When the pointer returned by the reentrant functions \fBgethostbyname_r()\fR,
\fBgethostbyaddr_r()\fR, and \fBgethostent_r()\fR is not \fINULL\fR, it is
always equal to the \fIresult\fR pointer that was supplied by the caller.
.sp
.LP
The \fBsethostent()\fR and \fBendhostent()\fR functions return \fB0\fR on
success.
.SH ERRORS
The reentrant functions \fBgethostbyname_r()\fR, \fBgethostbyaddr_r()\fR, and
\fBgethostent_r()\fR will return \fINULL\fR and set \fIerrno\fR to \fBERANGE\fR
if the length of the buffer supplied by caller is not large enough to store the
result. See \fBIntro\fR(2) for the proper usage and interpretation of
\fBerrno\fR in multithreaded applications.
.sp
.LP
The reentrant functions \fBgethostbyname_r()\fR and \fBgethostbyaddr_r()\fR set
the integer pointed to by \fIh_errnop\fR to one of these values in case of
error.
.sp
.LP
On failures, the non-reentrant functions \fBgethostbyname()\fR and
\fBgethostbyaddr()\fR set a global integer \fIh_errno\fR to indicate one of
these error codes (defined in \fB<netdb.h>\fR): \fBHOST_NOT_FOUND\fR,
\fBTRY_AGAIN\fR, \fBNO_RECOVERY\fR, \fBNO_DATA\fR, and \fBNO_ADDRESS\fR.
.sp
.LP
If a resolver is provided with a malformed address, or if any other error
occurs before \fBgethostbyname()\fR is resolved, then \fBgethostbyname()\fR
returns an internal error with a value of \(mi1.
.sp
.LP
The \fBgethostbyname()\fR function will set \fIh_errno\fR to
\fBNETDB_INTERNAL\fR when it returns a \fINULL\fR value.
.SH EXAMPLES
\fBExample 1 \fRUsing \fBgethostbyaddr()\fR
.sp
.LP
Here is a sample program that gets the canonical name, aliases, and ``.''
separated Internet \fBIP\fR addresses for a given ``.'' separated \fBIP\fR
address:

.sp
.in +2
.nf
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>
int main(int argc, const char **argv)
{
	in_addr_t addr;
	struct hostent *hp;
	char **p;
	if (argc != 2) {
	    (void) printf("usage: %s IP-address\en", argv[0]);
	    exit (1);
	}
	if ((int)(addr = inet_addr(argv[1])) == -1) {
	    (void) printf("IP-address must be of the form a.b.c.d\en");
	    exit (2);
	}
	hp = gethostbyaddr((char *)&addr, 4, AF_INET);
	if (hp == NULL) {
	    (void) printf("host information for %s not found\en", argv[1]);
	    exit (3);
	}
	for (p = hp->h_addr_list; *p != 0; p++) {
	    struct in_addr in;
	    char **q;
	    (void) memcpy(&in.s_addr, *p, sizeof (in.s_addr));
         (void) printf("%s\et%s", inet_ntoa(in), hp\(mi>h_name);
	    for (q = hp->h_aliases; *q != 0; q++)
	        (void) printf(" %s", *q);
	    (void) putchar('\en');
	}
	exit (0);
}
.fi
.in -2

.sp
.LP
Note that the preceding sample program is unsafe for use in multithreaded
applications.

.SH FILES
.ne 2
.na
\fB\fB/etc/hosts\fR\fR
.ad
.RS 22n
hosts file that associates the names of hosts with their Internet Protocol (IP)
addresses
.RE

.sp
.ne 2
.na
\fB\fB/etc/netconfig\fR\fR
.ad
.RS 22n
network configuration database
.RE

.sp
.ne 2
.na
\fB\fB/etc/nsswitch.conf\fR\fR
.ad
.RS 22n
configuration file for the name service switch
.RE

.SH ATTRIBUTES
See \fBattributes\fR(7) for descriptions of the following attributes:
.sp

.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE
_
MT-Level	T{
See \fBReentrant Interfaces\fR in the \fBDESCRIPTION\fR section.
T}
.TE

.SH SEE ALSO
.BR Intro (2),
.BR Intro (3),
.BR byteorder (3C),
.BR inet (3C),
.BR netdb.h (3HEAD),
.BR netdir (3NSL),
.BR hosts (5),
.BR netconfig (5),
.BR nss (5),
.BR nsswitch.conf (5),
.BR attributes (7)
.SH WARNINGS
The reentrant interfaces \fBgethostbyname_r()\fR, \fBgethostbyaddr_r()\fR, and
\fBgethostent_r()\fR are included in this release on an uncommitted basis only
and are subject to change or removal in future minor releases.
.SH NOTES
To ensure that they all return consistent results, \fBgethostbyname()\fR,
\fBgethostbyname_r()\fR, and \fBnetdir_getbyname()\fR are implemented in terms
of the same internal library function. This function obtains the system-wide
source lookup policy based on the \fBinet\fR family entries in
\fBnetconfig\fR(5) and the \fBhosts:\fR entry in \fBnsswitch.conf\fR(5).
Similarly, \fBgethostbyaddr()\fR, \fBgethostbyaddr_r()\fR, and
\fBnetdir_getbyaddr()\fR are implemented in terms of the same internal library
function. If the \fBinet\fR family entries in \fBnetconfig\fR(5) have a ``-''
in the last column for \fBnametoaddr\fR libraries, then the entry for
\fBhosts\fR in \fBnsswitch.conf\fR will be used; \fBnametoaddr\fR libraries in
that column will be used, and \fBnsswitch.conf\fR will not be consulted.
.sp
.LP
There is no analogue of \fBgethostent()\fR and \fBgethostent_r()\fR in the
netdir functions, so these enumeration functions go straight to the \fBhosts\fR
entry in \fBnsswitch.conf\fR. Thus enumeration can return results from a
different source than that used by \fBgethostbyname()\fR,
\fBgethostbyname_r()\fR, \fBgethostbyaddr()\fR, and \fBgethostbyaddr_r()\fR.
.sp
.LP
All the functions that return a \fBstruct hostent\fR must always return the
\fIcanonical name\fR in the \fIh_name\fR field. This name, by definition, is
the well-known and official hostname shared between all aliases and all
addresses. The underlying source that satisfies the request determines the
mapping of the input name or address into the set of names and addresses in
\fBhostent\fR. Different sources might do that in different ways. If there is
more than one alias and more than one address in \fBhostent\fR, no pairing is
implied between them.
.sp
.LP
The system attempts to put those addresses that are on the same subnet as the
caller before addresses that are on different subnets. However, if address
sorting is disabled by setting \fBSORT_ADDRS\fR to FALSE in the
\fB/etc/default/nss\fR file, the system does not put the local subnet addresses
first. See \fBnss\fR(5) for more information.
.sp
.LP
When compiling multithreaded applications, see \fBIntro\fR(3), \fBMULTITHREADED
APPLICATIONS\fR, for information about the use of the \fB_REENTRANT\fR flag.
.sp
.LP
Use of the enumeration interfaces \fBgethostent()\fR and \fBgethostent_r()\fR
is discouraged; enumeration might not be supported for all database sources.
The semantics of enumeration are discussed further in \fBnsswitch.conf\fR(5).
.sp
.LP
The current implementations of these functions only return or accept addresses
for the Internet address family (type \fBAF_INET\fR).
.sp
.LP
The form for an address of type \fBAF_INET\fR is a \fBstruct in_addr\fR defined
in <\fBnetinet/in.h\fR>. The functions described in \fBinet\fR(3C), and
illustrated in the \fBEXAMPLES\fR section, are helpful in constructing and
manipulating addresses in this form.
